The BBC recently profiled Gili Trawangan, a small island in Indonesia perhaps best known for these biorocks. The biorocks begin as naked metal sculptures of rebar and wire mesh, in shapes that include “giant steel manta rays, pyramids, planes, dolphins, whale sharks, lizards and turtles.” Once in the water, a weak current is passed through them. The electricity draws out ions dissolved in the water, such as calcium and magnesium, which form a hard coating around the metal.

The electrified biorocks essentially speed up the process of building reefs, which are naturally made of calcium carbonate. Divers also transplant injured coral to these structures, coaxing them back to health. As a coral reefs grow out of the metal, they become habitats for fish and small crustaceans.

With the technology working on waters near shore, the Gili Eco Trust is now looking to expand electric biorocks into the open ocean. The hang up? A power source for the electricity. To read about more the efforts to harness tidal power, check out the story at BBC Future.